Add documentation for the history_stats component (#1901)

* Add documentation for the history_stats component

* Remove time aliases from history_stats

* Change duration format

source/_components/sensor.history_stats.markdown

@@ -0,0 +1,136 @@
+---
+layout: page
+title: "History Statistics"
+description: "Instructions about how to integrate history_stats sensors into Home Assistant."
+date: 2017-02-10 12:00
+sidebar: true
+comments: false
+sharing: true
+footer: true
+logo: home-assistant.png
+ha_category: Sensor
+ha_iot_class: "Local Polling"
+ha_release: "0.38"
+---
+
+The `history_stats` sensor platform provides quick statistics about another component, using data from the history.
+
+It can track how long the component has been in a specific state, in a custom time period.
+
+Examples of what you can track :
+- How long you were at home this week
+- How long the lights were ON yesterday
+- How long you watched TV today
+
+## {% linkable_title Configuration %}
+
+To enable the history statistics sensor, add the following lines to your `configuration.yaml`:
+
+```yaml
+# Example configuration.yaml entry
+sensor:
+  - platform: history_stats
+    name: Lamp ON today
+    entity_id: light.my_lamp
+    state: 'on'
+    start: '{% raw %}{{ now().replace(hour=0).replace(minute=0).replace(second=0) }}{% endraw %}'
+    end: '{% raw %}{{ now() }}{% endraw %}'
+```
+
+Configuration variables:
+
+ - **entity_id** (*Required*): The entity you want to track
+ - **state** (*Required*): The state you want to track
+ - **name** (*Optional*): Name displayed on the frontend
+ - **start**: When to start the measure (timestamp or datetime).
+ - **end**: When to stop the measure (timestamp or datetime)
+ - **duration**: Duration of the measure
+
+
+
+<p class='note'>
+    You have to provide **exactly 2** of `start`, `end` and `duration`.
+<br/>
+    You can use [template extensions](/topics/templating/#home-assistant-template-extensions) such as `now()` or `as_timestamp()` to handle dynamic dates, as shown in the examples below.
+</p>
+
+## {% linkable_title Time periods %}
+
+The `history_stats` component will execute a measure within a precise time period. You should always provide 2 of the following :
+- When the period starts (`start` variable)
+- When the period ends (`end` variable)
+- How long is the period (`duration` variable)
+
+As `start` and `end` variables can be either datetimes or timestamps, you can configure almost any period you want.
+
+
+### {% linkable_title Duration %}
+
+The duration variable is used when the time period is fixed. Different syntaxes for the duration are supported, as shown below.
+
+```yaml
+# 6 hours
+duration: 06:00
+```
+
+```yaml
+# 1 minute, 30 seconds
+duration: 00:01:30
+```
+
+```yaml
+# 2 hours and 30 minutes
+duration:
+  # supports seconds, minutes, hours, days
+  hours: 2
+  minutes: 30
+```
+
+### {% linkable_title Examples %}
+
+Here are some examples of periods you could work with, and what to write in your `configuration.yaml`:
+
+**Today**: starts at 00:00 of the current day and ends right now.
+```yaml
+    start: '{% raw %}{{ now().replace(hour=0).replace(minute=0).replace(second=0) }}{% endraw %}'
+    end: '{% raw %}{{ now() }}{% endraw %}'
+```
+
+**Yesterday**: ends today at 00:00, lasts 24 hours.
+```yaml
+    end: '{% raw %}{{ now().replace(hour=0).replace(minute=0).replace(second=0) }}{% endraw %}'
+    duration:
+      hours: 24
+```
+
+**This morning (6AM - 11AM)**: starts today at 6, lasts 5 hours.
+```yaml
+    start: '{% raw %}{{ now().replace(hour=6).replace(minute=0).replace(second=0) }}{% endraw %}'
+    duration:
+      hours: 5
+```
+
+**Current week**: starts last Monday at 00:00, ends right now.
+
+Here, last Monday is _today_ as a timestamp, minus 86400 times the current weekday (86400 is the number of seconds in one day, the weekday is 0 on Monday, 6 on Sunday).
+```yaml
+    start: '{% raw %}{{ as_timestamp( now().replace(hour=0).replace(minute=0).replace(second=0) ) - now().weekday() * 86400 }}{% endraw %}'
+    end: '{% raw %}{{ now() }}{% endraw %}'
+```
+
+**Last 30 days**: ends today at 00:00, lasts 30 days. Easy one.
+```yaml
+    end: '{% raw %}{{ now().replace(hour=0).replace(minute=0).replace(second=0) }}{% endraw %}'
+    duration:
+      days: 30
+```
+
+**All your history** starts at timestamp = 0, and ends right now.
+```yaml
+    start: '{% raw %}{{ 0 }}{% endraw %}'
+    end: '{% raw %}{{ now() }}{% endraw %}'
+```
+
+<p class='note'>
+    If you want to check if your period is right, just click on your component, the `from` and `to` attributes will show the start and end of the period, nicely formatted.
+</p>